The classical example of a cytoplasmic localization in vertebrate eggs, believed responsible for the differentiation of a specific cell type, is the germ plasm. Studies on the presumed destruction of germ plasma by UV irradiation suggest a direct role of this material in the formation of primordial germ cells. One objective of our research using cytological procedures, is to determine whether irradiation actually destroys the "germ cell determinants" or whether it prevents germ cell formation by less direct means. The conventional view of gene activity during oogenesis suggests that oocytes, particularly at the lampbrush chromosome stage, are characterized largely by their ability to synthesize and sequester for later use, messenger RNA as well as other RNA species. The alternative view is that oocytes synthesize stable and unstable RNA's, as somatic cells, and that the content of maternal message represents a steady state level, attained early in oocyte growth, and maintained by continual synthesis and degradation throughout oogenesis. A second objective is to test this hypothesis by providing a detailed analysis of RNA metabolism, as well as the nuclear-cytoplasmic distribution of newly-synthesized RNA, in Xenopus laevis oocytes at several stages of growth. Our final objective concerns elucidation of the mechanism by which steroids induce oocyte maturation in amphibians. We propose a series of studies on the effects of progesterone on ion fluxes and membrane potential. We also will attempt to isolate and characterize a cytoplasmic maturation promoting factor which in itself will induce maturation when injected into recipient oocytes.